1. Field of the Invention
This invention relates to a method of discharge processing of a semiconductor.
2. Description of the Prior Art
The prior art methods of processing of semiconductors are roughly divided into two classes. One is a mechanical processing such as cutting. Another is photolithography method for forming electronic circuits. There are a lot of processing methods of photolithography. For example, removing processes such as etching, attaching processes such as deposition, and processing methods of changing characteristic of a substance such as ion injection are known. Though it is easy to form a very fine pattern by these processing techniques, it is difficult to process a semiconductor material with a high aspect ratio because these techniques are only methods for transferring the pattern on a plane to the surface of the semiconductor material as similar to photograph technique. For example, when a hole is formed in a semiconductor material, it is difficult to form a fine hole with a good straightness by etching because side etching occurs in the etching processing at the same time. On the other hand, processing of a semiconductor by discharge processing capable of processing a semiconductor with a high aspect ratio has been tried ("processing of Si wafer by discharge", the 1987 fiscal year papers of a scientific lecture at the spring meeting of the Japan society for precision engineering, p741). However, in such discharge processing, a resistivity of the semiconductor material needs to be lowered to the extent of 10.sup.-2 .OMEGA.cm. This is a problem in forming electronic circuits at its peripheral portions. In the case that a resistivity of a material to be processed is higher than this, the processing voltage needs to be increased with the resistivity ("Electric processing handbook", The Nikkan Kogyo shinbun Ltd.). Increase in processing voltage causes problems such as decrease in accuracy.
As mentioned above, the prior art photolithography such as etching is difficult for such processing. Moreover, in the prior art discharge processing, there are drawbacks as follows:
The first is that kinds of material used as a semiconductor material are limited because the resistivity should be low. Further, it is difficult to form an electronic circuit on such low-resistivity material. The second is that the accuracy in processing decreases and the size of a layer whose characteristic is changed by processing becomes large because a high voltage is used, so that energy for processing is large. Further, there is also a drawback that use of a high voltage more than 100 V can cause problems in circuits formed on the semiconductor material.
Moreover, semiconductor sensors and actuators having sizes of millimeters or micrometers are utilized in the following fields:
They are used for sensing of touch in robots with high functions or with actuators, for driving it. Moreover, they are also used for detection in a catheter for medical use, i.e., for diagnosis and medical treatment with it inserted in the body. Further, they are used as a pressure sensor in the fuel injection apparatus of an automobile engine.
In devices for detecting mechanical quantities using semiconductor materials such as the conventional pressure sensors, touch sensors, acceleration sensors, etc., etching techniques mainly used for processing of forming mechanical portions such as a diaphragm or a flat spring. Anisotropic etching processing is used for forming a thin plate portion such as the diaphragm or the flat spring, etc. The anisotropic etching is a method for processing using a characteristic that etching speed varies with crystal orientations or a density of impurities. FIG. 10 is a perspective view of a prior art dynamic sensor as an example of the conventional semiconductor mechanical device (Japanese patent publication No. 1-284764).
Moreover, a motor whose diameter is 100 micrometers as an ultra-micro actuator was produced. The method of its production uses thin film forming technique such as etching technique or deposition.
The ultra-micro sensor or actuator made by the production method using the above-mentioned prior art semiconductor processing technique has advantage effects as follows:
Ultra-fine forming processing is possible. Successive processing is possible by batch processing. Assembling of mechanical parts is easy. Moreover there are advantage effects such that it is possible to form a circuit for converting an electric signal indicative of the detected quantity of the sensor in the same substrate.
However, there are also drawbacks in the forming method of the prior art semiconductor processing technique of sensors and actuators as follows:
The first is that the product is limited to flat forms on a substrate. The second is that though there is the above-mentioned anisotropic etching as a technique for producing a form with a high aspect ratio, there is limitation in orientation of the material. The third is that if through-holes should be formed in the direction of thickness in parallel to each other in the ultra-micro sensors or actuators , it is necessary to provide a relatively large distance between them. The fourth is that in the actuator, the field of application is limited if the form is flat as mentioned above because the method of outputting and size thereof are important.
More specifically, the producing method of the prior art semiconductor processing technique particularly in the sensor will be described as follows with reference to FIGS. 11A and 11B:
FIGS. 11A and 11B are vertical and horizontal cross-sectional views of a prior art sensor for detecting shearing stress of a liquid. Refer SURFACE MICROMACHINING OF POLYIMIDE/METAL COMPOSITES FOR A SHEAR-STRESS SENSOR, Schmidt and another, 1987, IEEE Micro Robots and Teleoperators Workshop for this detection sensor.
In such detection sensors, because it is impossible to provide other circuits or projecting and hollow portions on the sensing surfaces of the sensor, circuits should be formed on the back thereof. Thus, it is necessary to pass a signal by forming a through-hole in the thickness direction. However, it is difficult to pass a lot of signals to the back at high density because a diameter of a through-hole is large according to the prior art semiconductor proceeding technique.